SQL double many-to-many relations with only two tables

Having two tables, one of them is called Tasks and the other one Relations where every task in Tasks might have 0-to-many predecessors and similarly, 0-to-many successors that relations among tasks are stored in the Relations-table.

So, if Task_1 is predecessor of Task_2 and Task_2 is predecessor of Task_3 then I would like to store the following records in the Relation-table:

for instance, using Django’s model definition:

class Tasks(models.Model):
    name_text = models.CharField(max_length=200)
    duration_int = models.IntegerField(default=1)

class Relations(models.Model):
    predecessors = models.ManyToManyField(Tasks)
    successors = models.ManyToManyField(Tasks)

Unfortunately, it does not work. I believe, that it is because giving the definition for successors as another relation to Tasks what already does exist might be inappropriate; although I also believe that Relation-table shall cascade back to Task-table with both of the relation: with predecessor and with the successors too in order to ensure the integrity.

Would any of you please help me out with this problem?

>Solution :

The modeling makes not much sense, you use two ForeignKey fields ^[Django-doc] for the predecessor and the successor:

class Task(models.Model):
    name_text = models.CharField(max_length=200)
    duration_int = models.IntegerField(default=1)
    successors = models.ManyToManyField(
        Task,
        through='Relation',
        through_fields=('predecessor', 'sucessor'),
        related_name='predecessor',
    )


class Relation(models.Model):
    predecessor = models.ForeignKey(
        Task, on_delete=models.CASCADE, related_name='successor_relations'
    )
    successor = models.ForeignKey(
        Task, on_delete=models.CASCADE, related_name='successor_relations'
    )